Throughout the world there is increasing interest in converting renewable biomass to usable products such as ethanol. Conversion of wood to ethanol has been practiced during wartime due to a shortage of liquid fuels. Reported in Ind. & Eng. Chem. Vol. 38 No. 9, P. 890 (1946). Because of poor yields and consumption of chemicals the conversion was found not to be economical for peacetime use. Present day interest in hydrolysis of biomass is to provide an alternative fuel source to avoid dependence on unreliable imported petroleum crude oil for liquid fuels. Biomass often contains hemicellulose and lignins accompanying the cellulose contained in the biomass. The hemicellulose and amorphous cellulose of the biomass is easily hydrolyzed, in a process termed pre-hydrolysis, leaving a residue containing lignins and un-hydrolyzed crystalline cellulose. Pre-hydrolysis consists of reacting water with biomass materials in the presence of a catalyst, usually an acid. Control of temperature of the hydrolysis and concentration of the acid is adjusted to control the kinetic rate of hydrolysis. Conventional processes center around acidic hydrolysis of biomass to produce glucose and pentoses. Pentoses, by continued hydrolysis conditions, may be reacted to form furfural. Biomass is a term used to describe material containing cellulose including: paper, pulp, wood waste, sawdust, municipal solid waste (MSW), agricultural wastes, fabrics, and cotton.
A state of the art hydrolysis process, described in U.S. Pat. No. 5,411,594, uses two stages to hydrolyze biomass materials The first stage is operated at low temperature and low pressure where both hemicelulose and amorphous cellulose are readily hydrolyzed. Residue from the first stage, containing lignins and difficult to hydrolyze cellulose, is removed and sent to a second stage operating at high pressure and high temperature for hydrolysis of crystalline cellulose. The hydrolyzate from the high pressure and high temperature stage contains acid and glucose and glucose degradradation products, 5-hydroxymethylfurfural (HMF). HMF is formed rapidly from glucose at high temperatures thus limiting the yield of glucose. Kinetic reaction rate of HMF is dependent on concentration of glucose and temperature.
Another process for converting biomass to glucose, described in U.S. Pat. No. 4,237,226, uses two stages. The first stage operating at low temperature and dilute acid will pre-hydrolyze hemicellulose and amorphous cellulose contained in biomass. Residue, containing crystalline cellulose and acid, from the first stage is neutralized then added to a second stage to hydrolyze the cellulose with added enzymes. This slow-acting hydrolysis process requires use of sterilized biomass residue in sterilized reactors. The cost of the enzymes is much more than that for acids.
A state of the art process being developed at TVA, reported in FY 1997 BIOCHEMICAL conversion/ALCOHOL FUELS PROGRAM: Annual Report PA.-65. Upon pre-hydrolysis of corn stover, with dilute acid in a first stage, residue from the first stage is treated with additional acid and then the residue is dried at low temperature to concentrate the acid contained in the residue. The resulting residue contains concentrated acid and crystalline cellulose which is rapidly hydrolyzed in a second stage reactor operating at low temperature. Un-hydrolyzed crystalline cellulose is contained in a residue with lignins.
Also, a state of the art process being developed by workers at the University of Arkansas, is reported in FY 1997 BIOCHEMICAL conversion/ALCOHOL FUELS PROGRAM: Annual Report PA.-85. It employs high concentration of sulfuric acid to convert corn stover to sugars. They describe a process scheme to separate sugars contained in the concentrated sulfuric acid using a heavy boiling solvent to dissolve the sulfuric acid and a low boiling solvent to dissolve the heavy boiling solvent. They also report that this method has a loss of solvents and a loss of sulfuric acid which is neutralized with lime.
Reported in the above named report, on page A-15, is a plan by TVA to develop a high concentration of sulfuric acid process. The current focus of TVA is to develop an inexpensive process for recovering the high concentration of sulfuric acid. Thus the problem of recovering sulfuric acid has not yet been solved.
It is well known that a high concentration of sulfuric acid will hydrolyze cellulose and hemicellulose at low temperatures to form water soluble depolymerized products which can be altered by enzymes to produce sugars for fermentation. The problem with this method is the failure to cost effectively recover the sulfuric acid. Recovery of the sulfuric acid is reported as an unsolved problem. Thus it is believed that no satisfactory recovery method has yet been developed.
The present concern is about recovering aqueous acidic liquor used to produce depolymerized cellulose and hemicellulose. This invention relates to a method of recycling aqueous acidic liquor used to produce depolymerized cellulose and hemicellulose contained in a biomass.
Therefore an object of this invention is to obviate many of the limitations and disadvantages of the prior art to hydrolyze cellulose and to ultimately produce glucose.
Depolymerized cellulose formed from aqueous acidic liquor may be hydrolyzed at low temperature to form glucose rapidly and therefore will not have time to form significant quantities of HMFs so as to reduce the lost yield of glucose.
Another object of this invention is to avoid use of costly and slow acting enzymes for hydrolysis of cellulose.
Still another object of this invention is to produce a yield of hydrolysis of cellulose and hemicellulose nearing 100%.
Yet another object of this invention is to operate and accomplish low energy consumption.
An additional object of this invention is to provide recycling of aqueous acidic liquor used to produce depolymerized cellulose and hemicellulose contained in a biomass.
With the above and other objects in view, this invention relates to the novel features and alternatives and combinations presently described in the brief description of the invention.